1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount, which utilizes a vacuum pressure for exhibiting different vibration damping and isolating characteristics based on the flow of a non-compressible fluid contained therein, depending upon the type of vibrations applied thereto. More particularly, the present invention is concerned with an improvement over the fluid-filled elastic mount, which efficiently utilizes as the vacuum pressure an intake pressure in an intake or suction system of an internal combustion engine of a motor vehicle while assuring a sufficiently high degree of durability of the mount.
2. Discussion of the Prior Art
An elastic mount, such as an engine mount or suspension bushing for a motor vehicle, is known as a vibration damping device interposed between two members of a vibration system, for flexibly connecting these two members. As a type of this elastic mount, there is known a so-called fluid-filled elastic mount as disclosed in JP-A-57-9340 and JP-A-63-172035. The elastic mount disclosed in these publications includes a first and a second support structure which are spaced apart from each other and are elastically connected to each other by an elastic body interposed therebetween. The elastic mount has a plurality of fluid chambers formed therein, which are filled with a suitable non-compressible fluid and are held in fluid communication with each other through an orifice passage or passages. Upon application of vibrations, the elastic mount of this type provides an intended vibration damping effect based on flow of the non-compressible fluid through the orifice passage.
However, the fluid-filled elastic mount of the above type provides a sufficiently high damping effect based on the fluid flow through the orifice passage, only with respect to input vibrations in a limited frequency range around the resonance frequency of the fluid in the orifice passage. Therefore, the above elastic mount is not suitably used as an engine mount or suspension bushing for a vehicle which is required to exhibit excellent vibration damping characteristics for a wider frequency range of vibrations.
In view of the above situations, it has been recently proposed to provide a fluid-filled elastic mount of a vacuum control type, as disclosed in JP-A-61-59035, JP-A-61-119834 and JP-A-61-153035. The elastic mount disclosed in the publications includes a flexible rubber membrane partially defining at least one of the fluid chambers, and has an air-tight operating chamber formed on one side of the flexible rubber membrane remote from the fluid chamber. In operation, the operating chamber is selectively subjected to a vacuum pressure so as to control elastic deformation of the rubber membrane, so that the elastic mount exhibits different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto. In particular, this elastic mount is advantageously employed in a mounting system including an internal combustion engine of a motor vehicle, for example, since the elastic mount can readily utilize an intake pressure in an intake pipe of the engine, as the vacuum pressure to be applied to the operating chamber.
However, the inventors' study on the known elastic mount described just above revealed that a combustion gas (air-fuel mixture) containing gasoline or its vapor, which flows from an engine combustion chamber back into the intake pipe, undesirably flows into the operating chamber of the mount where the operating chamber is held in direct communication with the intake pipe. Consequently, the flexible rubber membrane bounding the operating chamber is likely to be affected by the pre-combustion gas flow from the engine, resulting in considerably lowered durability of the mount.
In the meantime, the engine mount, which is used at greatly varying ambient temperatures, favorably employs as the non-compressible fluid filling the fluid chambers, alkylene glycol and the like which exhibit relatively low viscosity over a wide temperature range. However, a rubber material conventionally used for the flexible membrane of the engine mount, for example, does not have sufficiently high degrees of resistance to both the fluid in the fluid chambers and gasoline in the pre-combustion gas flowing into the operating chamber. Therefore, it has been difficult to assure a sufficiently high degree of durability of the fluid-filled elastic mount of the above-described vacuum control type.